 | Northern Idaho
Fertilizer Guide Current Information Series No. 911 |
V. J. Parker-Clark and R. L. Mahler
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| Northern Idaho
Fertilizer Guide Current Information Series No. 911 |
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Lawns are a very important part of our landscape. Besides being aesthetically pleasing, they cover erodible soils, produce oxygen, and fit nicely into our forested areas. However, lawns can be very expensive to care for if they are treated incorrectly. Incorrect fertilization can result in diseased lawns, weedy lawns, or lawns that adversely impact water quality through the leaching of applied nitrates into groundwater.
To fertilize your lawn correctly, you need to understand its nutrient needs. Lawns need four macronutrients: nitrogen (N), phosphorus (P), potassium (K), and sulfur (S). Lawns growing in neutral or high pH soils (pH values greater than 6.8) may also need the micronutrient iron (Fe).
There are two kinds of N fertilizer: slow release and quick release. Slow-release fertilizers become available slowly. Use them in sandy soils, in other soils that drain rapidly, or when grass plants are not growing rapidly -- early spring and fall. Slow-release N fertilizers are often referred to as WIN (water insoluble nitrogen) materials. Quick-release fertilizers provide readily available N to plants. Quick-release fertilizers are best to use when the grass is rapidly growing in early summer.
Phosphorus (P)
Phosphorus promotes strong root growth and encourages lawns to thicken
quickly. Phosphorus levels are often low in northern Idaho soils.
Because P does not move through the soil as N does, you should apply it
when you establish the lawn or immediately after aerating it. If the
soil is highly erodible, P will run off with sediment.
Potassium (K)
Adequate K is necessary for disease resistance. It also allows the lawn
to stand up to heavy traffic and promotes winter hardiness. Potassium
is usually adequate in our soils. Too much K can result in an
accumulation of salts.
Sulfur (S)
Sulfur is needed in most of northern Idaho. Be sure to include it in
your fertilizer mix.
Soil testing strategy
Soil tests for lawn fertilization should be done in early spring,
within a month of the time you are planning the first application of
fertilizer. To get a representative sample of soil in your lawn, take
12 to 15 subsamples, mix them together, and take a quart sample from
the mixed sample.
Each subsample should come from the top 6 to 8 inches of soil. Make sure you exclude any surface debris that can change the soil test results, including grass blades. The more subsamples you take, the more representative your sample will be. Avoid taking samples from gardens, from under shrubs, or from unusual areas in the yard. If the soil in your lawn varies from one location to another, consider treating each unique area as a separate sample. For more information on how to collect and process a soil sample, see University of Idaho Extension Bulletin 704, Soil Sampling. A standard soil test will also give you the pH of the soil and the percentage of organic matter. Soil test information and the following tables will help you determine the correct amount of each nutrient to apply.
Nitrogen -- Determine the N application rate per 1,000 square feet of lawn based on the soil's organic matter content (Table 1).
| Soil test organic matter | N
supplying capacity of your soil | N application
rate per 1,000 ft2 |
|---|---|---|
| (%) | (lb) | |
| 7 or higher | Very high | 1 |
| 5 to 7 | High | 2 |
| 3 to 5 | Moderate | 3 |
| 1 to 3 | Low | 4 |
| under 1 | Very low | 5 |
Phosphorus -- Determine the P application rate per 1,000 square feet of lawn based on a P soil test (Table 2). Soil-testing laboratories use two different methods of determining soil P -- sodium bicarbonate (NaHCO3) and sodium acetate (NaOAc). Make sure you read the column in Table 2 that matches the method your soil testing laboratory used.
| Soil test P | P-supplying capacity | Application rate | |
|---|---|---|---|
| NaHCO3 | NaOAc | of your soil | per 1,000 ft2 |
| (ppm) | (ppm) | (lb phosphate [P2O5]) | |
| 12 or higher | 5 or higher | Very high | None |
| 9 to 12 | 4 to 5 | High | None |
| 6 to 9 | 3 to 4 | Moderate | None |
| 3 to 6 | 2 to 3 | Low | 1 |
| 0 to 3 | 0 to 2 | Very low | 2 |
Potassium -- Determine the K application rate per 1,000 square feet of lawn based on a K soil test (Table 3).
| Soil test K | K-supplying
capacity of your soil | Application
rate per 1,000 ft2 |
|---|---|---|
| (ppm) | (lb potash [K2O]) | |
| 250 or higher | Very high | none |
| 150 to 250 | High | 1 |
| 100 to 150 | Moderate | 2 |
| 50 to 100 | Low | 4 |
| less than 50 | Very low | 4 |
Sulfur -- Apply S if your soil tests less than 10 parts per million (ppm) SO4-S. An application rate of 1 pound S per 1,000 square feet should be adequate for an entire year.
Micronutrients -- Northern Idaho lawns generally do not need micronutrients. The only potentially deficient micronutrient is iron (Fe). Iron is usually deficient only when the soil pH is greater than 6.8. Iron deficiency symptoms -- "chlorosis" or the yellowing of new growth -- can be corrected by applying a 1/2 percent solution of ferrous sulfate as a foliar spray or a chelated iron at the recommended label rate. Applications may need to be repeated if yellowing reoccurs.
Nutrient ratio strategy
The nutrient ratio fertilization strategy does not involve a soil
test. It is based on applying 0.5 pounds of N per 1,000 square feet
of lawn for each month of active grass growth. (When daily
temperatures average above 80 F, most grasses are not actively growing
unless you water them. Most lawns in northern Idaho start active
growth in early to mid-April and often continue to grow until
mid-October). If, for example, your lawn grows actively 8 months each
year, you would apply 4 pounds of N per 1,000 square feet over the
year.
Phosphorus, K, and S applications are based on a ratio of those nutrients to the amount of N applied: three parts N, to one part P, to two parts K, to one part S. Thus, if your N recommendation is 4 pounds per 1,000 square feet, your P recommendation would be 1.3 pounds, your K recommendation would be 2.6 pounds, and your S recommendation would be 1.3 pounds.
Let's say you have a lawn that is actively growing 6 months each year.
You would calculate N, P, K, and S fertilizer needs for the year as
follows:
0.5 lb N per 1,000 ft2 per month x 6 months = 3.0 lb N per
1,000 ft2
3N = 3.0 lb N
1P = 1.0 lb P
2K = 2.0 lb K
1S = 1.0 lb S
You would buy a lawn fertilizer with a N:P:K:S ratio of 3:1:2:1 and apply as directed under "Fertilizer Application." Because you may not be able to obtain a fertilizer with a 3:1:2:1 ratio exactly, select a fertilizer with a ratio as close to it as possible.
For example, if you need 3 pounds of N per 1,000 square feet, you
would apply it as follows:
0.75 lb N around Easter
0.75 lb N around Memorial Day
0.75 lb N around Labor Day
0.75 lb N around Halloween
Do not apply more than 1 pound N per 1,000 square feet at one time.
The container label will have three or four numbers called the fertilizer grade, 16-20-0-15, for example. The first number is always the percentage of N in the bag, the second number is the percentage of P2O5, the third is the percentage of K2O, and fourth is the percentage of S or some other nutrient that is specified on the label. A fertilizer with a grade of 15-10-10-2 contains 15 percent N, 10 percent P2O5 (phosphorus pentoxide), 10 percent K2O (potassium oxide), and 2 percent S.
Select a brand that supplies the nitrogen (N), phosphate (P2O5), and potash (K2O) in approximately the same ratio as your soil test indicates or in the 3:1:2:1 ratio used in the ratio strategy. If the soil test recommends that you apply 1 pound of actual N per 1,000 square feet, 3 pounds of phosphate, and no potassium, this is a 1-3-0 ratio. You could use a material like 13-39-0-7, which would give you 7 percent sulfur.
Table 4 gives the amount of fertilizer material required to supply 1 pound of actual plant nutrient. The table lists fertilizer products commonly available in northern Idaho.
To calculate the amount of fertilizer to apply, use the following
equation:
| lb nutrient recommended per 1,000 ft2 x
100 __________________________________ = % nutrient in fertilizer material |
lb fertilizer needed per 1,000 ft2 |
Example 1:
To supply 1 lb N per 1,000 ft2 using ammonium nitrate
(34-0-0):
| 1 x 100 _______ = 34 | 2.9 lb per 1,000 ft2 of 34-0-0 |
Example 2:
To supply 1 lb N per 1,000 ft2 as urea (45-0-0) and 0.5 lb
P2O5 per 1,000 ft2 using triple
superphosphate (0-44-0):
| for N, | 1 x
100 _________ 45 | = 2.2 lb per 1,000 ft2 of 45-0-0 |
| for P, | 0.5 x
100 ___________ 44 | = 1.1 lb per 1,000 ft2 of 0-44-0 |
Example 3:
To supply 0.6 lb N per 1,000 ft2 as IBDU (31-0-0), 0.4 lb
P2O5 per 1,000 ft2 using single
superphosphate (0-20-0), and 1.0 lb K2O per 1,000
ft2 using potassium chloride (0-0-60):
| for N, | 0.6 x
100 ___________ 31 | = 1.9 lb per 1,000 ft2 of 31-0-0 |
| for P, | 0.4 x
100 ___________ 20 | = 2.0 lb per 1,000 ft2 of 0-20-0 |
| for K, | 1.0 x
100 ___________ 60 | = 1.7 lb per 1,000 ft2 of 0-0-60 |
For additional information on fertilizer terminology, calculations, and application practices, see University of Idaho CIS 863, Fertilizer Primer: Terminology, Calculations and Application.
If the soil pH is greater than 7, add 25 pounds of elemental S per 1,000 square feet. Note that only elemental S, not sulfate-sulfur (SO4-S), will lower soil pH. Soil amendments are best applied in the fall.
Use only the amount of N that the soil test or ratio fertilization strategy indicates. Water in the morning between 6 a.m. and noon. Water deeply a couple of times a week instead of shallowly every day or every other day. Monitor soil moisture and the amount of water needed to wet the top 6 inches of soil.
Total nitrogen | Available phosphate | Water- soluble potash | Total sulfur | Material needed to supply 1 pound of plant nutrient | |||||
|---|---|---|---|---|---|---|---|---|---|
| Fertilizer materials | (N) | (P2O5) | (K2O) | (S) | (N) | (P2O5) | (K2O) | (S) | |
| (%) | (%) | (%) | (%) | (lb) | (lb) | (lb) | (lb) | ||
| Inorganic | |||||||||
| Ammonium nitrate | 34 | -- | -- | -- | 3.0 | -- | -- | -- | |
| Ammonium sulfate | 21 | -- | -- | 24 | 4.8 | -- | -- | 4.1 | |
| Urea | 46 | -- | -- | -- | 2.2 | -- | -- | -- | |
| Urea formaldehyde | 38 | -- | -- | -- | 2.6 | -- | -- | -- | |
| Urea ammonium nitrate solution | 32 | -- | -- | -- | 3.1 | -- | -- | -- | |
| Sulfur coated urea (SCU) | 31-36 | -- | -- | 21 | 3.0 | -- | -- | 4.8 | |
| Isobutylidene diurea (IBDU) | 31 | -- | -- | -- | 3.2 | -- | -- | -- | |
| Monoammonium phosphate | 11 | 48-55 | -- | -- | 9.1 | 2.0 | -- | -- | |
| Diammonium phosphate | 16-18 | 46-48 | -- | -- | 5.8 | 2.1 | -- | -- | |
| Single super-phosphate | -- | 18-20 | -- | 12 | -- | 5.0 | -- | 8.3 | |
| Triple super-phosphate | -- | 44-46 | -- | 1 | -- | 2.2 | -- | 100.0 | |
| Potassium chloride | -- | -- | 60-62 | -- | -- | -- | 1.7 | -- | |
| Potassium sulfate | -- | -- | 50-53 | 18 | -- | -- | 2.0 | -- | |
| Sulfate of potash magnesia | -- | -- | 22 | 22 | -- | -- | 4.5 | 4.5 | |
| Organic | |||||||||
| Bone meal | 1-4 | 2-8 | 0 | 0 | 50.0 | 22.0 | -- | -- | |
| Fish meal | 10.0 | 2.6 | 0 | 0 | 10.0 | 38.4 | -- | -- | |
| Manures | 1-4 | 0.2-2 | 1-2 | 0 | -- | ||||
| Wood ash | 0 | 0.9 | 5.0 | 0 | -- | 111.0 | 20.0 | -- | |
| Complete | |||||||||
| 12-6-6 | 12 | 6 | 6 | 0 | 8.3 | 16.6 | 16.6 | -- | |
| 9-3-6-3S | 9 | 3 | 6 | 3 | 11.1 | 33.3 | 22.2 | 33.3 | |
| 12-4-8-4S | 12 | 4 | 8 | 4 | 8.3 | 25.0 | 12.5 | 25.0 | |
| 24-12-12 | 24 | 12 | 12 | 0 | 4.2 | 8.3 | 8.3 | -- | |
| 22-4-4-12S | 22 | 4 | 4 | 12 | 4.5 | 25.0 | 25.0 | 8.3 | |
| 14-24-14-4S | 14 | 24 | 14 | 4 | 7.1 | 4.2 | 7.1 | 25.0 | |
| 16-6-8-18S | 16 | 6 | 8 | 18 | 6.3 | 16.7 | 12.5 | 5.5 | |
| 32-4-8-12S | 32 | 4 | 8 | 12 | 3.1 | 25.0 | 12.5 | 8.3 | |
| CIS 583 | Selecting Turfgrasses for Idaho Lawns (35 cents) |
| CIS 731 | Thatch in Lawns (25 cents) |
| CIS 792 | Calibration and Safe Use of Lawn and Garden Pesticide and Fertilizer Applicators (35 cents) |
| CIS 888 | Weed Control in Lawns (50 cents) |
| EXP 676 | Fairy Rings in Turf (50 cents) |
| EXT 723 | Herbicides for Lawn Weed Control (50 cents) |
The authors -- Vickie J. Parker-Clark, University of Idaho Extension agricultural agent in Kootenai County, and Robert L. Mahler, professor of soil science, University of Idaho, Moscow.
Issued in furtherance of cooperative extension work in
agriculture and home economics, Acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture, LeRoy D. Luft,
Director of Cooperative Extension System, University of Idaho, Moscow,
Idaho 83844. The University of Idaho provides equal opportunity in
education and employment on the basis of race, color, religion,
national origin, gender, age, disability, or status as a Vietnam-era
veteran, as required by state and federal laws.
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Comments to author: karenl@uidaho.edu
All contents copyright © 1996-2002. College of Agricultural and Life Sciences, University of Idaho. All rights reserved. Revised: January 3, 2002